1. Field of the Invention
The present invention relates to a novel self-adjusting control for power management of an ultrasonic welding system. More particularly, the present invention relates to an improved ultrasonic generator and controller useful in automatic wire bonders for producing consistent wire bonds under different load conditions.
2. Description of the Prior Art
Automatic wire bonders are known and are employed to automatically make fine wire interconnections between pads on a semiconductor chip and the leads of a carrier or lead frame to provide an electrically interconnected semiconductor device. Wires are thermocompression bonded using force and heat and are thermosonically bonded using force, heat and ultrasonic scrub energy. Wire bonds can also be made without using heat by applying force and ultrasonic scrub energy to produce an ultrasonic bond. The predominant method of bonding is known as ball bonding using automatic thermosonic bonding systems. Such systems are made and sold by Kulicke and Soffa Industries, Inc. of Willow Grove, Pa. and have reached a very high degree of computer automation resulting in repeatable accuracy of force and movement of the bonding capillary used to make wire bonds.
Heretofore such automatic wire bonders were provided with ultrasonic generators which produced controlled power to the transducer which holds the bonding capillary that makes the wire bonds. Heretofore transducer power has been controlled by either maintaining constant current (I.sub.c) or constant voltage (V.sub.c). U.S. Pat. No. 4,808,948 is directed to an automatic tuning system for ultrasonic generators and recognizes that the resonant frequency of a particular ultrasonic transducer which holds the bonding capillary changes its resonant frequency under load and this patent suggests that automatically tuning the voltage control oscillator (VCO) in the phase lock loop will adjust the VCO frequency to the resonant frequency of the transducer. This automatic tuning of ultrasonic generators solves part of the wire bonding problem, however, it did not discuss the problem that the actual power coupled to the bonding target load (Z.sub.L) through the wire under the capillary bonding tool varies as a result of such changes.
The present invention starts with the assumption that an optimum condition does not exist from bond to bond, thus, any attempt to set a predetermined optimum fixed power level at the ultrasonic generator will not produce the same Z.sub.L power through the wire that is being bonded.
Heretofore it has been recognized that the power output of the transducer used in an ultrasonic welding system varies from transducer to transducer. Manufacturers of such transducers have attempted to make designs and improve manufacturing methods so that the transducers are more uniform and the impedance and resonant frequency will fall within closely defined specifications. Such attempts to refine the manufacture and specifications of ultrasonic transducers for automatic wire bonders has appreciably increased the cost of manufacturing and quality control inspection.
It is the purpose of the present invention to determine the amount of power that theoretically should be coupled through the wire being bonded and to supply that necessary power to the ultrasonic transducer to produce a desired power level to the load impedance (Z.sub.L) at the wire being bonded under varying load conditions at each wire bond. Not only does this approach diminish the need for the small manufacturing differences that are inherent in the systems, but it also strongly diminishes the differences that exist due to load conditions. Heretofore, it was known that the resonant frequency of a transducer varied with the mounting system, the bonding tool, the work holder clamps, roughness, rigidity, coupling ability, transducer temperature, humidity, drift and aging among other conditions. These same conditions also affect the power requirements, thus, it is impossible to predict the power needed as an input to any transducer under load for different wire bond conditions.
It would be desirable to provide an ultrasonic generator/controller that would determine the amount of power needed at the wire being bonded under high speed continuous production conditions and to automatically supply the power needed without interfering with the speed of an automatic wire bonded during production.